A computed tomography scanner (CT) usually comprises a so-called gantry, with an X-ray beam source (X-ray tube) and a radially opposite (X-ray) detector being attached thereto. The gantry is intended to rotate around an object to be irradiated, with the X-ray radiation emitted and modified during the penetration of the object being detected by the detector.
The detector is generally assembled from a plurality of individual detector modules. Each detector module comprises a sensor unit which in turn has a scintillator for converting the X-ray radiation into visible light and photodiodes for detecting this light.
The scintillator is normally formed from a plurality of scintillator pixels, usually in the form of cubic elements of scintillating material. The scintillator pixels are generally arranged in an array, that is to say in a matrix arranged in a checkerboard fashion in rows and columns. Narrow interspaces (so-called septa), filled with a light reflecting and/or absorbing material, are in each case formed between the individual pixels; these septa are used to delimit the individual pixels in respect of one another in terms of radiation.
A method for producing such a scintillator is disclosed in, for example, DE 198 49 772 A1. Accordingly, elongate scintillator elements are firstly layered next to and above one another in parallel and bonded to form a block whilst forming the septa. Subsequently, the block is cut into slices, transversely with respect to the longitudinal direction of extent of the scintillator elements, such that the individual slices have the desired array structure.
Within the scope of the sensor unit, each scintillator pixel has one photodiode associated with it. In a conventional sensor unit—as is described, for example, in DE 10 2005 014 187 A1—these photodiodes are arranged in an array matched to the pixel structure of the scintillator, with the array being fitted to the outer side of the scintillator facing away from the X-ray tube.
Each sensor unit is often also assigned a collimator which is used to suppress scattered beams of the X-ray radiation impinging on the scintillator. The collimator is usually formed from a stack of thin tungsten sheets which are attached to a support plate in an upright fashion in respect of the latter. This support plate is usually fitted to an inner side of the scintillator intended to face the X-ray tube. Here, the collimator sheets are respectively arranged approximately flush with the septa, as a result of which the X-ray radiation is incident on the scintillator pixels in a directed fashion.
Since the collimator sheets should not shadow the scintillator pixels in the process, very exact positioning of the former in respect of the scintillator is necessary. Exact positioning of the collimator sheets is difficult, particularly in the case of very small pixels and very narrow septa of the scintillator.